Conventional fluid level sensors utilize mechanical, ultrasonic, and/or electrical components to measure and indicate the level of fluid in a vessel. Mechanical methods, such as floats, are susceptible to breakdown and are often bulky in size. Ultrasonic methods require acoustic reflection from the liquid surface and need electrically excited transducers. Electrical techniques, such as resistance or capacitance measurements, employ metallic wires and components immersed in the fluid, which may be undesirable in the presence of corrosive liquids or inflammable fluids where electrical sparks can be hazardous. In such environments it may be advantageous or necessary to use non-metallic components to measure fluid levels. As an example, the measurement of the level of aviation fuel in aircraft fuel tanks may be accomplished more safely with components and cables that do not conduct electricity. Thus, for reasons of safety, accuracy, and reliability, there is a need for a non-metallic device which uses optical energy and non-moving components to measure fluid levels.
The few optical techniques currently available for detecting fluid levels are generally based upon an on/off response at the discrete end of an optical fiber placed in the fluid. In one such typical sensor, light entering the fiber is reflected back from a prism-shaped tip of the fiber if the fluid is below the tip, but the light is transmitted into the liquid (and therefore little is reflected) when the fluid level is above the tip. A serious limitation of this technique is its inability to measure the level of the fluid other than to determine whether it is above or below the fiber tip.
To overcome this limitation, several fibers have been bundled together as described by John Rakucewicz in "Fiber-Optic Methods of Level Sensing," Sensors, p. 5 (Dec. 1986). However, this method increases the sensor size, complicates the fluid detection, and is limited to detection of a few discrete fluid levels depending on the number of individual fibers.
Other optical approaches currently being investigated include devices in which the amplitude of light returned from the immersed sensor varies as a function of the level of fluid. These methods usually use narrowband light (such as from a laser or light emitting diode). However, these amplitudebased devices suffer from inaccuracies caused by instabilities in the light output of the sensor, drift of the detector, and variable losses in the transmitting and receiving fibers. Consequently, onsite calibration and frequent recalibration is often necessary to maintain accuracy of amplitude-based sensors.
Thus, a need has been identified for an optical fluid level sensor that overcomes the deficiencies of amplitude-based optical sensors. In particular, the desired optical sensor should be insensitive to the composition or chemical nature of the fluid being sensed, should be permanently calibrated, and should be unaffected by variations and losses in the intensity of the light.